LTE-Advanced (Long-Term Evolution) is currently standardised in 3GPP (3rd Generation Partnership Project). The LTE-Advanced standard corresponds to the release 10 of LTE. In release 10 it has been decided to support “Type 1” relay nodes (RN). A type one relay is characterised by some important characteristics. A type one relay controls cells, each of which appears to a UE (User Equipment) as a separate cell distinct from the donor cell. The cells shall have their own Physical Cell ID (as defined in LTE Rel-8) and transmit their own synchronization channels, reference symbols etc. In the context of single-cell operation, the UE receives scheduling information and HARQ (Hybrid Automatic Repeat-reQuest) feedback directly from the RN and send its control channels to the RN. A type one relay shall appear as a Rel-8 eNB (enhanced Node B) to Rel-8 UEs, i.e. be backwards compatible. This means basically that from a UE perspective, there is no difference being served by an eNB or a type 1 relay.
The type 1 relays communicate with a donor eNB and one or several UEs. Between the relay and the eNB, transmissions are done on the backhaul link. Transmission between the UE and the relay are done on the access link and the UE and the eNB communicates via the direct link.
If the transmissions on the backhaul links and the access links in the system are performed within the same frequency band, the relays are referred to as inband relays. To enable inband relays to be functional there is a need to separate the transmissions and receptions at the relay, i.e. the relay cannot transmit and receive at the same time on the same frequency, since this could cause intolerable interference. For this purpose, during a certain subframe, the UEs associated to the relay do not expect to receive any DL (DownLink) data from the relay. Instead, these subframes are used for carrying data from the donor-eNB to the RNs.
UE mobility, when connected to a RN, is handled in the same way as when the UE is connected to the eNB. When the UE is in active mode the RN controls the cell level mobility of the UE with help of the measurement configuration and UE measurement reporting, and by triggering the handover procedure. When the UE is in idle mode the cell selection is controlled by the UE based measurements.
Handover or cell selection between cells is usually done based on downlink RSRP (Reference Signal Received Power). The UE connects to the eNB from which it receives the strongest signal. Alternatively, the cell selection decision can be based on RSRQ (Reference Signal Received Quality) measurements, where also the current interference situation is taken into account.
In normal deployments, the RSRP and/or RSRQ measurements used for mobility provide a good basis to support the correct cell selection decision. The reason is that the RSRQ and RSRP can be mapped reasonably well to an expected data rate in the target cell. For a system with inband relays, this is no longer valid, i.e. the RSRP and/or RSRQ cannot be directly mapped to the experienced data rate of the UE. The quality of the backhaul link as well as the MBSFN (Multimedia Broadcast multicast service Single Frequency Network) configuration, i.e. backhaul subframe configuration, are factors that will affect the end to end throughput that is experienced by the UE when it connects to the RN. The end-to-end bit rate for a user served by a relay is limited by the link that supports the lowest bit rate, depending on the backhaul allocation. It is clear that, in certain situations, a non-negligible portion of the UEs connected to relays would be better off if they were directly connected to the eNB despite a lower RSRP and/or RSRQ. Cell selection is crucial in order to secure improved performance in e.g. a relay enhanced system. The existing tools and algorithms for an optimized cell selection procedure are not sufficient in a relay based deployment.